Theory question: IAC/PCV

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2 weeks 1 day ago #81624 by Cameron76
I can't wrap my head around this.  I've been researching for hours and can't find a solid answer.  So I'll try the forum. I think there is a detail in these situations I am missing.  By the way, thank you to Paul and his administrators for taking the time to answer everyone's questions.

A PCV valve stuck open seems like it would look exactly like a stuck open IAC at idle, since blow-by gasses are very low at idle. 

In a MAF engine, a stuck open IAC would allow too much air into the intake, but that air is metered by the MAF since the IAC is downstream of the MAF. A stuck open PCV valve would also allow too much air into the intake, but again that air is metered, since the fresh air inlet is downstream of the MAF.  So it seems that injection pulse would be matched by the PCM for the volume of air metered by the MAF in both a stuck open PCV and stuck open IAC.  

In a MAP engine, a stuck open PCV valve would be constantly dumping air into the intake but would be seen (metered) by the MAP sensor as an increased load.  A stuck open IAC would again be dumping unwanted air into the intake manifold but that air would be seen (metered) by the MAP sensor as an increased load. 

I understand that at idle, you wouldn't want all that extra fuel and air, so the engine would run rough, and maybe stall.  The combustion process would probably send a bunch of unburned O2 and raw fuel down the tailpipe.  And an O2 only registers the presence and absence of oxygen, so it would give a lean code with the unburned O2. 

Is this accurate? Or am I missing something?
 

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2 weeks 23 hours ago - 2 weeks 23 hours ago #81632 by Tyler
Replied by Tyler on topic Theory question: IAC/PCV
I don't think you're missing anything. But unfortunately, I've only ever seen stuck open/excessive PCV flow on MAF systems, so it's tough for me to say with certainty about the MAP system.

A PCV valve stuck open seems like it would look exactly like a stuck open IAC at idle, since blow-by gasses are very low at idle. 

In a MAF engine, a stuck open IAC would allow too much air into the intake, but that air is metered by the MAF since the IAC is downstream of the MAF. A stuck open PCV valve would also allow too much air into the intake, but again that air is metered, since the fresh air inlet is downstream of the MAF.  So it seems that injection pulse would be matched by the PCM for the volume of air metered by the MAF in both a stuck open PCV and stuck open IAC.
 


Generally agree. The only variable would be if the PCV system is strong enough to suck air past the crankshaft seals. On some older domestic PCV systems, the metering orifice just doesn't produce that much suction, even at full flow. But many Euro makes/models, the PCV has the ability to suck air past seals, creating a vacuum leak and unmetered air.

In a MAP engine, a stuck open PCV valve would be constantly dumping air into the intake but would be seen (metered) by the MAP sensor as an increased load.  A stuck open IAC would again be dumping unwanted air into the intake manifold but that air would be seen (metered) by the MAP sensor as an increased load.


This sounds right? But again, never ran into it firsthand. I don't have any MAP vehicles to experiment on, either. 
Last edit: 2 weeks 23 hours ago by Tyler.

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